Just like paint the surface needs to be smooth because any imperfection will show. It sounds like you are getting winkles where the hood slopes forward. You may need to cut the decal where the wrinlkes form to allow them to lay on top of each other. this will help the decaal conform to the curve of the hood better. This is a problem with covering curved and irregular surfaces. Another way to solve the problem is to cut darts (a small V shapped cut) in the decal. This will also allow the decal to follow the curve of the surface better.
I use a flat tipped brush to apply solvent and work the decals. The brush is a Floquil #4 flat tipped brush. If it a large flat surface I use a paper towel to squeeze out as much of the trapped air before using solvent.
The templates I have problems with are for odd shaped parts like MotoGP bike airboxes and fenders. I have the same problems as you. The decal gets wrinkles where I should have used two decals or slit the decal. I'm still learning what works.